Guide bushes mounted on the column of an automatic lathe to hold a rod-like workpiece rotatably at a position near a cutting tool are classified into a rotary type and a stationary type. The rotary guide bush rotates together with a workpiece and holds the workpiece for axial sliding while the stationary guide bush remains stationary and holds a workpiece for rotation and axial sliding without rotating itself.
The guide bush of either type has a taper outer surface, a portion provided with slits for affording elasticity to the portion, a threaded portion for mounting the guide bush on the column, and an inner surface for holding a workpiece. The inner surface constantly in sliding contact with the workpiece is susceptible to wear and, particularly, the inner surface of the stationary guide bush undergoes intense wear.
A guide bush proposed in, for example, JP-A No. 4-141303 has an inner surface, coming in sliding contact with a workpiece making sliding and rotative movement, securely attached with a superhard alloy or a ceramic material by brazing or the like.
When the inner surface of a guide bush is attached with a superhard alloy or a ceramic material excellent in wear resistance and heat resistance, the wear of the inner surface of the guide bush can be reduced to some extent.
However, when the workpiece is subjected to heavy machining on an automatic lathe, in which the depth of cut is large and the cutting speed is high, the workpiece is damaged or seizure occurs due to a decrease in a clearance in the radial direction between the guide bush and the workpiece even if the inner surface of the gude bush is attached with the superhard alloy or the ceramic material because the superhard alloy and the ceramic material have a comparatively large coefficient of friction and a low thermal conductivity. Therefore, it has been difficult to increase the depth of cut and cutting speed.
The stationary guide bush has advantages in that a workpiece can be accurately machined in a high roundness because the workpiece can be held such that its axis has no runout, less noise is generated, and the construction of the automatic lathe may be made simpler and compact.
However, the inner surface of the stationary guide bush is worn far more rapidly than that of the rotary guide bush and hence, it has been difficult to increase further the depth of cut and the cutting speed when the stationary guide bush is employed.
For solving such problems, a proposal has been made to provide a guide bush wherein wear resistance of an inner surface thereof, in sliding contact with a workpiece, is dramatically enhanced by forming a hard carbon film over the inner surface, enabling an automatic lathe to machine a workpiece with an increased depth of cut and at a higher cutting speed without damaging the workpiece or causing seizure between the guide bush and the workpiece.
The hard carbon film is formed of hydrogenated amorphous carbon having properties closely resembling those of diamond. Therefore, the hydrogenated amorphous carbon is also called diamond like carbon (DLC).
The hard carbon film (DLC film) has high hardness (not lower than Vickers 3000 Hv), excellent wear resistance, a small coefficient of friction (about 1/8 that of a superhard alloy), and excellent corrosion resistance.
For example, a plasma CVD process as described hereinafter is available as a method of forming the hard carbon film over the inner surface of the guide bush.
More specifically, in carrying out the process, a vacuum vessel is evacuated so as to reach a degree of vacuum not higher than 3.times.10.sup.-5 torr (such a pressure as reached initially is referred hereinafter as "an initially reached pressure") after disposing the guide bush inside the vacuum vessel.
Then, a carbon-containing gas is fed into the vacuum vessel, and the pressure therein is regulated to be 5.times.10.sup.-3 torr, which is a film-forming pressure.
Subsequently, a DC voltage at -3 kV is applied to the guide bush and a DC voltage is applied to an anode, disposed opposite to the guide bush, and an AC voltage is applied to a filament, respectively, causing a plasma to be produced inside the vacuum vessel, and forming a hard carbon film over the surface of the guide bush by the agency of the carbon plasma.
However, with the plasma CVD process described above, the hard carbon film is formed primarily by the agency of the plasma produced in a region surrounding the guide bush, decomposing the carbon-containing gas, and therefore, a problem will arise that although the hard carbon film can be formed uniformly over the outer surface of the guide bush, the hard carbon film formed over the inner surface of the guide bush has a poor adhesion property and is inferior in film qualities such as hardness and the like.
This is attributable to a phenomenon that an unintended electric discharge, called a hollow cathode discharge, is caused to occur in the center bore of the guide bush by the plasma produced therein because the center bore occupies a space wherein electrodes at the same electric potential face each other. The hard carbon film formed by the hollow cathode discharge is a polymer-like film poor in adhesion property, prone to be easily exfoliated from the inner surface of the guide bush, and having low hardness.
Further, in the method of forming the hard carbon film as described above, a DC voltage at -3 kV is applied to the guide bush from a DC power source when the pressure is at 5.times.10.sup.-3 torr, which is the film-forming pressure.
In such a condition wherein the pressure in the vacuum vessel is on the order of 5.times.10.sup.-3 torr, the space within the vacuum vessel is full of electric charges such as electrons, lowering the impedance of the space. As a result, the moment a plasma discharge starts, an unintended electric discharge such as an arc discharge is apt to occur.
Further, since the time when the plasma discharge starts corresponds to an initial stage of the hard carbon film formation, adherence of the hard carbon film to the guide bush is dependent on the quality of the film formed at the initial stage of the hard carbon film formation.
Consequently, if the unintended electric discharge, such as the arc discharge, occurs at the outset of the plasma discharge, the quality and adherence of the hard carbon film will be degraded, causing a problem of the film being exfoliated from the inner surface of the guide bush.
It is therefore an object of the invention to provide a method capable of forming a hard carbon film of high quality and excellent adhesive property over the inner surface of the guide bush, which will come in sliding contact with workpieces, by solving the problems described above.